This invention relates to a laser-based processing machine incorporating a gas-flushed beam guiding chamber and an optical element providing the end of the beam guiding chamber on the beam input side and serving to direct the laser beam into the beam guiding chamber and to the pressurized beam guiding chamber. A flushing gas intake opening is provided in the beam guiding chamber near the optical element. It also relates to a method for protecting an optical element located in the gas-flushed laser beam guiding chamber against contaminants.
Such laser-based processing machines are known and have been described in the patent literature, e.g., EP 0 749 800.
A laser beam impinging on an optical element is not totally reflected or transmitted; instead, a small part of it is absorbed which leads to a heat buildup in the optical element. Any fouling of the optical element by external contaminants increases the level of absorption of the laser beam. This additionally absorbed laser power can damage the optical element, shortening its useful life while also reducing the effective laser power.
In the laser processing machine described in EP 0 749 800, air with a specific CO2 content is fed into a tubular beam guide. The air serves the purpose of flushing the tubular beam guide, thus keeping the laser beam free of performance-reducing gases and particles. The air is fed into the tubular beam guide through an air intake near the optical window through which the laser beam is directed from the laser resonator into the tubular beam guide. The wall at the other end of the tubular beam guide is provided with an air outlet opening equipped with an adjustable diaphragm serving to regulate the amount of air discharged and thus to control the positive pressure within the tubular beam guide.
In the laser processing of materials, for instance when cutting or welding with a laser, the processing result depends on the power density and beam quality of the laser beam. To obtain the desired processing results, it is often necessary to adjust precisely the radius of the laser spot on the workpiece. Apart from its laser mode, a laser beam contains diffraction components which have a larger beam radius and distant field divergence pattern than does the laser mode. When an optical element focuses the laser beam on a metal plate that is to be processed, the diffraction components are situated outside the laser-mode beam radius, leading to an undesirable buildup of heat outside the beam radius of the laser mode. Therefore, prior art laser systems incorporate optical diaphragms for beam forming and especially for filtering out the diffraction components.
It is the object of the present invention to provide a novel and improved laser guide chamber assembly containing an optical element in which the amount of flushing gas needed for protecting the optical element from laser beam affecting contaminants is minimized.
It is also an object to provide such a laser guide chamber assembly which is relatively simple in construction and reliable in operation.
It has now been found that the foregoing and related objects may be readily attained in a laser processing machine including a gas flushed beam guiding chamber, and an optical element providing an end of the beam guiding chamber on the beam input side and serving to direct the laser beam into the beam-guiding chamber. A flushing gas intake opening is provided in the beam guiding chamber near the optical element, and an optical diaphragm aperture is provided in the beam guiding chamber for shaping the laser beam and also serving as a restrictor for flow of the flushing gas therethrough.
In one embodiment the laser processing machine includes a focussing device which focuses the laser beam in the direction of the diaphragm aperture, and this focussing device may be integrated into the optical element. Generally, the focussing device is positioned within the beam guiding chamber. Preferably, the diaphragm is positioned at a distance within the Rayleigh length from the neck of the laser beam focussed by the focussing device.
Desirably, there is included a control device for adjusting the diaphragm. The diaphragm aperture is circular and its diameter is smaller than                     4        ⁢                  2                ⁢        f        ⁢                  xe2x80x83                ⁢        λ                    π        ⁢                  xe2x80x83                ⁢        d        ⁢                  xe2x80x83                ⁢        K              ⁢          xe2x80x83        ⁢          when the diaphragm is positioned at the neck of the laser beam        or                    8        ⁢        f        ⁢                  xe2x80x83                ⁢        λ                    π        ⁢                  xe2x80x83                ⁢        d        ⁢                  xe2x80x83                ⁢        K              ⁢          xe2x80x83        ⁢          when the diaphragm is positioned within the Rayleigh length      
Wherein f is the focal length of the focussing device, d is the diameter of the laser beam, and K is the characteristic factor of the beam.
The intake opening directs the flushing gas flowing into the beam guiding chamber at the optical element parallel to the surface of the optical element Alternatively, the intake opening directs the flushing gas flowing into the beam guiding chamber away from the optical element. In the most customary embodiment, the aperture and the restrictor are provided by a cutting gas nozzle on the cutting head of a laser processing machine and the flushing gas also serves as a processing gas in the laser cutting process.
The salient advantage of the laser processing machine according to this invention lies in the fact that the diaphragm subdivides the beam guiding chamber and only the part of the beam guiding chamber between the optical element and the diaphragm needs to be under positive pressure. The amount of gas needed for flushing is correspondingly reduced. The flushing gas is preferably fed into the beam guiding chamber through a gas inlet equipped with a flow control device. In this fashion, the pressure within the beam guiding chamber can be so regulated to build up positive pressure, and with the diaphragm aperture constitutes a restrictor port for the flushing gas flowing through the beam guiding chamber.
The laser beam is preferably focussed onto the diaphragm by means of a focussing device and is bent at the diaphragm aperture. Especially in the case of a long laser beam path the beam radius can be held small and the laser beam can be directed to the focussing device with only minor losses. The focussing device may be positioned inside the beam guiding chamber or integrated into the optical element. In the latter case, the beam guiding chamber can be of a correspondingly smaller design. This also reduces light losses which occur in optical elements in any imaging process. In conjunction with the focussing device that is positioned behind the diaphragm, the focussing device which concentrates the laser beam toward the diaphragm may be combined into a Kepler telescope.
The diaphragm does not have to be located at the focal point of the laser beam, and it may be positioned for instance in front of the focal point which further reduces the part of the beam guiding chamber that needs to be pressurized. In preferred design versions, the diaphragm is located within the Rayleigh length from the neck of the laser beam focussed by the focussing device. In the area of the beam neck the spatial separation between the laser mode and the diffraction components is the largest, providing at this point an opportunity to filter out the diffraction components, while minimizing the losses in the laser mode.
A particular advantage is derived from making both the diaphragm and the diaphragm aperture adjustable. The position of the diaphragm and the diaphragm aperture can thus be adapted to the location of the neck of the focussed laser beam, to the laser power, to the processing job involved, and to the temperature of the diaphragm. Especially in materials processing applications it is necessary to precisely select the beam radius at the processing point in order to obtain the desired processing result.
In the case of a circular diaphragm aperture the diameter of the aperture is preferable smaller than, for instance,                     4        ⁢                  2                ⁢        f        ⁢                  xe2x80x83                ⁢        λ                    π        ⁢                  xe2x80x83                ⁢        d        ⁢                  xe2x80x83                ⁢        K              ⁢          xe2x80x83        ⁢          when the diaphragm is positioned at the neck of the laser beam        or                    8        ⁢        f        ⁢                  xe2x80x83                ⁢        λ                    π        ⁢                  xe2x80x83                ⁢        d        ⁢                  xe2x80x83                ⁢        K              ⁢          xe2x80x83        ⁢          when the diaphragm is positioned within the Rayleigh length      
where f is the focal length of the focussing device, xcex is the wavelength of the laser, d is the diameter of the laser beam at the focussing device and K is the characteristic factor of the beam.
After verification of the necessary purity level of the flushing gas such as helium, nitrogen or other inert gases, the gas flow can be initiated and directed at the optical element or in parallel fashion past the optical element into the beam guiding chamber. Otherwise, for instance when using compressed air or the filtered exhaust air from a vacuum pump that draws laser gas from the laser resonator, the flushing gas should be introduced away from but as close as possible to and past the optical element into the beam guiding chamber. The orientation of the gas flow entering the beam guiding chamber relative to the optical element can be adjusted for instance by means of a nozzle provided at the intake opening.
In particularly preferred design versions of this invention, the aperture and the restrictor port are in the form of a cutting gas tip at the processing tool head of the laser processing machine. The cutting gas tip doubles as the optical aperture for the laser beam and the flushing gas also serves as a processing gas in the laser processing operation, for instance in purging liquid substances from the groove or slot the laser beam has cut into a sheet-metal workpiece.
As part of the method first above mentioned, this particular objective is achieved by setting the pressure of the gas flowing through and flushing the beam guiding chamber in such fashion that an optical diaphragm aperture provided in the beam guiding chamber for shaping the laser beam simultaneously operates as the restrictor for the flushing gas.
This invention also relates to the concurrent use of an optical aperture for a laser beam as a restrictor port for a flushing gas, and to the use of a cutting-gas tip as an optical aperture for a laser beam and as a restrictor port for a flushing gas that also serves as a processing gas.
Other advantages will be evident from the following description and from the drawings. According to the invention, the features described above and those discussed further below can be applied individually as discrete features or in any desired combination. The embodiments described and illustrated are not to be viewed as a finite and final enumeration but only as examples serving to explain this invention. In the drawingsxe2x80x94